1

Timers

setTimeout setInterval

2

Pending Callbacks

deferred I/O errors from previous iteration

TCP errors

3

Idle, Prepare

internal to Node

4

Poll

the workhorse

where most of your async code actually executes

1 First — drain the poll queue:

fs.readFile http request socket data dns.lookup

            then, if the queue is empty — in order:
          

2 if setImmediate is pending → jump to Check

3 else if a timer expired → next iteration Timers

4 else → block here and wait for new I/O

5

Check

runs right after Poll

setImmediate

6

Close Callbacks

socket.on('close'), server.close()

            I/O →
          

incoming: connections, data, etc.

            ↻ next tick
          

once all six phases drain, the loop starts over